With the advancement of information processing systems, functional materials performing various functions, such as nonlinear optical effects, photochromism and the like, have been searched for in pursuit of development of an optical theory element, which is a fundamental technique for an optical computer, an optical switch, an optical memory, a tunable laser, etc. Known functional materials in the field of the optical industry include inorganic ferroelectric substances, e.g., LiNbO.sub.3, BaTiO.sub.3, and KH.sub.2 PO.sub.4 ; quantum well semiconductors using GaAs, etc.; organic single crystals of 4'-nitrobenzylidene-3-acetamino-4-methoxyaniline (MNBA), 2-methyl-4-nitroaniline (MNA), etc.; low-molecular dyes, e.g., spiropyran and furylfulgide; and conjugated organic high molecular polymers, e.g., polydiacetylene and polyarylene-vinylene. Studies have been continued particularly on organic functional materials because of their structural diversity as compared with inorganic materials, and a number of organic functional materials have hitherto been proposed.
In general, inorganic materials involve complicated processes, such as sputtering and melt sintering and require large-sized equipment for making elements. On the contrary, organic materials do not require complicated equipment but meet difficulty in obtaining a sufficiently large single crystal for practical use. Besides, organic materials were difficult to fabricate into an element due to their poor mechanical strength.
It has been suggested to add a low-molecular dye to a high molecular polymer matrix. However, the effect of a low-molecular dye held in a high polymer matrix film does not last long because the low-molecular dye undergoes decomposition by the influences of impurities, and the like. Further, where an element using a high polymer matrix is connected to a glass waveguide exemplified by optical fiber, there occur problems, such as insertion loss arising from a difference in refractive index and difficulty in connecting the two. The waveguide loss in the element itself is larger than in glass materials.
In order to solve the above problems, it has been proposed to immobilize an organic functional compound in a silica gel matrix by a sol-gel process to thereby obtain a functional composite material composed of an inorganic material and an organic material excellent in light transmission and weathering resistance (see JP-A-2-188441, JP-A-3-33031 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), and J. Non-cryst. Solid, Vol. 135, p. 1 (1991)).
However, a tetraalkoxysilane which has been employed as a matrix-forming substance in a conventional sol-gel process is apt to develop cracks on drying of a gel, and a sol solution thereof, when coated on a substrate, fails to provide a film with a sufficient thickness. Where a dye is to be incorporated into an inorganic silica gel matrix using a tetraalkoxysilane, it is difficult to incorporate a sufficiently high concentration of a dye because addition of a high concentration of a dye results in agglomeration of the dye particles due to poor compatibility with the matrix. According to JP-A-2-188441, dispersibility of a dye in a sol solution may be improved by ultrasonic wave application. However, the dye still undergoes agglomeration since the solvent of the sol solution, such as an alcohol, vaporizes during gelation and drying of the sol solution. Therefore, the improving effect is still insufficient. Further, the dye is influenced by the strong polar field of a residual silanol group, a siloxane bond, etc. in a gel so that it cannot fully display its function. Addition of a surface active agent has been suggested as a means for improving compatibility between the inorganic matrix and an organic dye and for alleviating the influence of the polar field (see JP-A-3-33031). Further, J. Phys. Chem., Vol. 95, p. 976 (1991) reports that a surface active agent forms a micelle-like aggregate in a silica gel matrix, demonstrating reduction of polarity around a dye added thereto as a result of fluorometry using pyrene as a probe. However, a surface active agent is not always stable in a solid and tends to lose its effect by heat or with time.